1. Field of the Invention
This invention relates to an ophthalmological diagnosis method, particularly to an ophthalmological diagnosis method in which a laser beam of a prescribed diameter is used to illuminate the eye fundus and to produce a laser speckle pattern formed by the light scattered by blood cells in the tissues of the eye. A small, circular detecting aperture of a prescribed diameter is used to detect movement of the laser speckle pattern at an observation plane as fluctuations in the speckle light intensity, and the photon correlation function thereof is measured to determine the state of the blood flow in eye fundus tissues.
2. Description of the Prior Art
Conventional laser Doppler methods of measuring blood flow in retinal and other tissue by illuminating the eye fundus with a laser beam include those described in "Investigative Ophthalmology," vol. 11 No. 11, p936 (November 1972) and "Science," vol. 186 (November 1974) p830, and in Japanese Unexamined Patent Publication Nos. 55-75668, 55-75669, 55-75670, 52-142885 (corresponding to GB 13132/76 and U.S. Pat. No. 4,166,695), 56-125033 (corresponding to GB 79/37799), 58-118730 (corresponding to U.S. Pat. No. 4,402,601) and U.S. Pat. No. 4,142,796. However, these laser Doppler methods involve the use of a high precision optical system, are complicated to use and provide results which lack repeatability and reliability, which hinder the practical utilization of the method.
In order to overcome the aforementioned drawbacks the present inventors have adapted laser speckle methods used for blood flow measurement in skin and the like (such as the methods described in Japanese Unexamined Patent Publication Nos. 60-199430, 60-203235 and 60203236 and in "Optics Letters," vol. 10 No. 3 (March 1985) p104) for ophthalmological applications involving evaluating the state of the blood flow in tissues of the eye fundus, and have filed the following related patent applications: Japanese Unexamined Patent Publication Nos. 62-275431 (U.S. Pat. No. 4,734,107 and EPC 234869), 63-238843 (EPC 284248) and 63-242220 (EPC 285314).
In the methods described in these publications with respect to eye fundus measurements, a detecting aperture is used to extract time-base fluctuations in the intensity of speckles formed at an optical Fourier Transform plane with respect to the eye fundus, or at the Fraunhofer refraction plane, or at an image plane (or a magnified image plane) that is conjugate with respect to the eye fundus.
There is the necessity of projecting a laser beam as weak as possible in a short time for safety reasons. This results in the detection of a very weak intensity of light, thus needing a photon correlation method for correlation computation which uses a digital correlator to count photoelectric pulses in order to evaluate the blood flow state. In this case, measurement is made not only for a normal blood flow, but also for an abnormal fast or slow blood flow appearing in diseased eyes. The blood flow in the eye fundus also depends on a spot to be measured and on the patient to be examined. Thus, extraction of a good correlation curve greatly depends on how the sampling time .DELTA.t is set which determines a time resolution in carrying out the correlation calculation.
The digital correlator which has been used so far computes correlation data with a predetermined sampling time .DELTA.t, and thus cannot reconstruct the correlation data with a different sampling time .DELTA.t. This necessitates renewed sampling of data. Usually, it is hard to exactly evaluate which sampling time .DELTA.t is suitable for obtaining the correlation function before measurement is initiated. The sampling time must usually be changed every day depending on intended use. Thus, the repeated sampling of data burdens the patients and disadvantageously consumes time.
If a correlation curve indicates that the object has high and low frequency components, there further would occur a problem in the case where the evaluation is performed
with the sampling time .DELTA.t which is more suitably applicable for either one of the frequency components. This means that the measurement must be repeated with a different sampling time each time.
Furthermore, when a time change in blood flow at a certain time is to be observed, it is necessary to perform measurement at a sub-divided time. In this case, measurement data sampled at the sub-divided time are stored for analysis upon completion of measurement and measurement is then again initiated after completion of analysis. This results in intermittent and inaccurate measurement. The sub-division of the measurement time may thus be dependent on intended use.
On the other hand, as described in Japanese Unexamined Patent Publication No. 63-242220 (EPC 285314), the blood flow in a single specific blood vessel may be evaluated by detecting the speckle pattern on the image plane. In the method thus described, a detecting aperture is aligned with the image of the blood vessel of interest on a magnified image plane and time-course fluctuations in speckle light intensity at that image plane are extracted to obtain a speckle signal. With this arrangement, a frequent occurrence during the measurement procedure is that the fundus image moves out of alignment on the detection plane owing to any of a number of factors including eye movement, vibration and misalignment between the apparatus and the eye. This will often result in obstruction by the wall of the blood vessel or the intrusion of a different blood vessel into the detection zone. In a clinical context, these are major problems which are constantly encountered, especially when the eye is inadequately fixed or when the patient is particularly apprehensive.
A look at the correlation functions of signals obtained from a single measurement shows that the signals include various components, which has an adverse effect on the repeatability of blood flow measurements. Also, since it is unclear which components are not required or to what extent the inclusion of a particular component contributes to the end result, an attempt to extract the original blood flow signal by removing unnecessary components from a correlation function curve based on data from one measurement is an impracticably difficult task. In addition, a blink of the eye during the measurement process or the intrusion of noise or an undesired signal can make an entire set of correlation data obtained during a measurement session unusable for the purposes of accurate evaluation, requiring that the measurement be redone from the beginning.